This invention is concerned with improvements in or relating to adhesives and adhesive bonding.
In the field of adhesives, various proposals have been made with a view to avoiding fire and toxicity hazards associated with use of so-called solvent based adhesives. For example, proposals have been made to employ so-called hot melt adhesives for adhesive bonding of various materials. In the field of packaging for example hot melts are attractive from the point of view of speed of set up and ease of automatic or semi-automatic application. However, in some fields for example shoe manufacture, hot melt adhesives are not widely used partly because relatively high application temperatures have been required with consequent risk of degradation of the adhesive or potential damage to personnel or substrates to be bonded. Also, ready provision of adequate adhesive bonds does not appear possible with certain substrate materials commonly employed in the shoe trade using conventional hot melt adhesives.
An important requirement in providing adhesive bonds is that the composition applied must have sufficient fluidity to enable it adequately to wet the substrate to which it is applied. Preferably also the composition should effect at least limited penetration of fibrous substrates. Often it is also important that the composition has or develops a tough and strong condition in the bond. These requirements of adequate wetting and development of toughness and strength are notable in adhesively bonding shoe soles to shoe uppers, where the finished product may be expected to be subjected to variations in stress under various conditions of use. When using solvent based adhesives, this initial fluidity has been secured by the use of volatile organic solvent solutions of adhesives which develop strength and toughness after evaporation of the solvent.
An adhesive capable of application in solvent-free condition and which cures to form strong bonds to a variety of substrates (including those commonly employed in the manufacture of shoes) in an economically acceptable time, would be commercially highly desirable. Indeed, for many years it has been desirable to provide an adhesive applicable as a hot melt without risk of heat damage and which cures to provide strong sole attaching bonds on a variety of substrates.
In our U.K. Patent Specification No. 1540634 there is described and claimed a method of adhesively bonding a shoe sole to a shoe upper using selected urethane prepolymer compositions which are provided as fluid compositions and which are convertible by a chain extension treatment to a polyurethane which is tacky and capable of adhesive union when heated and so to establish substantially immediate strong adhesive union with a compatible adhesive or resin surface. The chain extension is preferably done at higher temperatures under humid conditions, e.g. in a steam cabinet or alternatively at room temperatures at relative humidities of 20 to 60%.
The method described in U.K. Patent Specification No. 1540634 has the advantage of employing a substantially solvent-free adhesive composition applied at a comparatively low temperature, but a significant disadvantage accompanying use of the preferred adhesive compositions there described is the considerable time needed for chain extension of the prepolymer after application. The uppers which are cemented with the prepolymer have to be cured for significantly long periods of time at comparatively high temperature and comparatively high humidity in order to give required chain extension sufficient to obtain a bond of acceptable initial strength. Under these conditions, the shoe uppers may become damaged by the high humidity. Lower relative humidity slows the rate of the chain extension reaction down to an extent which renders the process more unacceptable.
Attempts to improve the speed of chain extension for example by appropriate selection of raw materials have met with some success, but the practical requirements for tolerance of chain extension conditions and of delays prior to heat activation conditions used in a shoe factory, together with the need also for producing strong bonds to a variety of substrates pose a significant problem.
Characteristics desirable for an adhesive composition for application in at least substantially solvent-free form for sole attaching purposes include the following properties. The adhesive composition should be capable of application in fluid form at a temperature which is sufficiently low that the shoe making materials used are not adversely affected, and so that persons who may come in contact with the hot melt do not suffer burns to the skin. Thus, application temperatures of 100.degree. C. or less are highly desirable. In order to be applicable at such temperatures the adhesive composition should have at such temperatures viscosity and flow characteristics which enable it to properly wet surfaces to which it is applied and preferably also to penetrate fibrous materials for example leather, whereby to establish at an early stage in the process good conditions for forming adhesive bonds to the materials. When cooled, coatings of the adhesive present on the materials should harden adequately within a few minutes to provide at least a non-tacky surface, and preferably a solid form-retaining non-tacky coating on the material to the extent that soles and shoe uppers coated with the adhesive composition may be handled and stored for limited periods of time without sticking together and without significantly disturbing the applied layers of adhesive composition. Coatings of the adhesive composition on the sole should remain at least for a period of three days, and preferably for up to 60 days capable of heat activation, for example by exposure to a radiant heater set at 85.degree. C. to 95.degree. C., to a condition in which sole spotting can be carried out and of then establishing a substantially immediate adhesive union with the coating of adhesive composition on the shoe upper by pressing the sole and shoe upper together in a sole press. It is also desirable that the adhesive union achieved between the sole and shoe upper should initially be sufficiently strong to avoid springing of the sole edges from the shoe upper during handling and storage immediately after sole pressing, and should be or become adequate to withstand normal conditions of use of the shoe as epitomised by ability to withstand a peel test at 5 Kg per cm width carried out 72 hours after sole pressing.
When using an adhesive composition comprising a moisture curable polyurethane prepolymer, it is desirable that coatings of the adhesive composition on the sole should be susceptible to the effect of moisture in the atmosphere under normal, ambient, conditions of storage of the soles (i.e. approximately 20.degree. C..+-.5.degree. C. and relative humidity of not less than approximately 10%) to bring about conversion of the prepolymer to a tough, solid, but heat softenable condition. It is convenient if this condition is achieved during storage overnight and it is highly desirable that the conversion of the prepolymer of an adhesive composition coating on the sole does not progress during storage in such a way that the coating becomes incapable of being satisfactorily heat softened during a period of less than seven days after application, in view of a normal practice in shoe making whereby adhesive coated soles may be kept in storage under ambient conditions over a weekend or more prior to attachment to the shoe.
It is one of the objects of the present invention to provide an improved method of bonding a sole to a shoe upper.
The term "shoe" where used herein relates to outer footwear generally and includes the article in the course of manufacture. The term "sole" where used herein relates to shoe tread members generally and includes heels and sole and heel units.